This leads to FCNC processes, such as \(K \rightarrow \pi \mathcal {X}_L\) ,
\(B \rightarrow K \mathcal {X}_L\) , etc,
whose rate is enhanced as \((E/m_{\mathcal {X}})^{2}\) , where \(E\)  is the decay energy (cf. the derivative operator in Eq. (REF )),
thus implying strong bounds on light
vector bosons coupled to anomalous currents [1]}, [2]}.
